a) Field of the Invention
The present invention relates to optical elements such as a vari-focal lens element, a vari-focal diffractive optical element and a variable declination prism which are to be used as liquid crystal optical elements. The present invention also relates to an electronic image pickup unit which uses these optical elements.
b) Description of the Prior Art
For composing a vari-focal lens system of lens elements which are manufactured by polishing a glass material, it is conventional to change a focal length of the lens system by moving a lens unit(s) in a direction along an optical axis, for example, as in a zoom lens system for cameras since the lens elements cannot change focal lengths by themselves. However, such a lens system has a drawback that it has a complicated mechanical structure.
For correcting such a drawback, there has been proposed an optical system which uses a polarizing plate 1 and a liquid crystal lens component 2, for example, as shown in FIG. 1. The liquid crystal lens component 2 comprises lens elements 3a and 3b, and a liquid crystal layer 5 which is disposed between these lens elements by way of transparent electrodes 4a and 4b, and an AC power source 7 is connected between the transparent electrodes 4a and 4b by way of a switch 6, whereby the optical system is configured to change a refractive index of the liquid crystal layer 5 by selectively applying an electric field to the liquid crystal layer 5.
When natural light, for example, is incident on the polarizing plate 1 of this optical system, only a predetermined linearly polarized component transmits through the polarizing plate 1 and is incident on the liquid crystal lens component 2. In a condition where the switch 6 is turned off and no electric field is applied to the liquid crystal layer 5 as shown in FIG. 1, longer axes of liquid crystal molecules 5a are oriented in a direction of a plane of polarization of the incident linearly polarized component, whereby a refractive index of the liquid crystal layer 5 is enhanced and a focal length of the liquid crystal lens component 2 is shortened. In a condition where the switch 6 is turned on and an electric field is applied to the liquid crystal layer 5 as shown in FIG. 2, in contrast, the longer axes of the liquid crystal molecules 5a are oriented in parallel with an optical axis, whereby the refractive index of the liquid crystal layer 5 is lowered and the focal length of the liquid crystal lens component 2 is prolonged. The focal length of the optical system shown in FIG. 1 is variable by selectively applying an electric field in the liquid crystal lens component 2 as described above.
However, the optical system shown in FIG. 1 poses a problem that it attenuates rays to be incident on the liquid crystal lens component 2 during transmission through the polarizing plate 1 and lowers a light utilization efficiency since it requires to dispose the polarizing plate 1 before the liquid crystal lens component 2 so that only the predetermined linearly polarized component is incident on the liquid crystal lens component 2. Further, the optical system which utilizes light at such a low efficiency poses another problem that it is applicable only to limited instruments or has a low versatility.
Further, an electronic image pickup unit for electronic cameras, video cameras and the like consists of a combination of an image pickup device 8 and a lens system 9 as shown in FIG. 3.
Such an electronic image pickup unit generally uses a lens system which has a relatively complicated composition, has a complicated configuration as a whole, comprises a large number of parts and requires tedious assembly, thereby being limited in compact design and reduction of a manufacturing cost thereof.
In view of the conventional problems described above, a primary object of the present invention is to provide optical elements having variable optical characteristics, i.e., a vari-focal optical element, a vari-focal diffractive optical element, a vari-focal mirror and a variable declination prism usable as liquid crystal optical elements which are adequately configured so as to enhance light utilization efficiencies, be applicable efficiently to various kinds of optical instruments and has excellent versatility.
The vari-focal optical element according to the present invention is characterized in that it comprises: a first optical member which has first and second surfaces, and allows incident rays to transmit through the first and second surfaces; a second optical member having a third surface which receives rays having transmitted through the first optical member; a lens surface which is formed on at least one of the first, second and third surfaces; a pair of transparent electrodes disposed on the second surface and the third surface respectively; and a polymer dispersive liquid crystal layer which is disposed between these transparent electrodes, and that it is configured so as to be capable of changing a focused point of rays which have transmitted through the first and second optical members or rays which have transmitted through the first optical member, have been reflected by the third surface and have transmitted again through the first optical member by applying an electric field to the polymer dispersive liquid crystal layer by way of the pair of transparent electrodes.
Further, the vari-focal diffractive optical element according to the present invention is characterized in that it comprises: a first optical member which has first and second surfaces, and allows incident rays to transmit through the first and second surfaces; a second optical member which has third and fourth surfaces, and allows rays which have transmitted through the first optical member to emerge through the third and fourth surfaces; a diffractive surface which is formed at least one of the first, second and third surfaces; transparent electrodes which are disposed on sides of the second surface and the third surface respectively; and a polymer dispersive liquid crystal layer which is disposed between these transparent electrodes, and that it is configured so as to be capable of changing a focused point of rays which have transmitted through the first and second optical elements by applying an electric field to the polymer dispersive liquid crystal layer by way of the pair of transparent electrodes.
Furthermore, the variable declination prism according to the present invention is characterized in that it comprises: a first optical member which has first and second surfaces, and allows incident rays to transmit through the first and second surfaces; a second optical member having third and fourth surfaces, and allows rays which have transmitted through the first optical member to pass through the third and fourth surfaces; an inclined surface which is formed on at least one of the first, second and third surfaces; transparent electrodes which are disposed on sides of the second surface and the third surface respectively; and a polymer dispersive liquid crystal layer which is disposed between these transparent electrodes, and that it is configured so as to be capable of changing declinations of rays which have transmitted through the first and second optical members by applying an electric field to the polymer dispersive liquid crystal layer by way of the pair of transparent electrodes.